1. Field of the Invention
This invention relates generally to processing of plastic resin material in the course of fabrication, by molding or extrusion, into finished colored plastic parts, and more particularly to methods and apparatus for processing liquid color material used to impart a desired color to the finished plastic parts.
2. Description of the Prior Art
Color is imparted to finished plastic parts by adding a coloring agent, either liquid color material or solid color pellets, to plastic resin material as it is being fabricated into finished plastic parts. The liquid color or the colored pellets control the color of the finished plastic parts. Depending on whether liquid color or solid colored pellets are used, the position at which the coloring agent is added to the plastic resin material during the part fabrication process may change.
Colored solid pellets are easy to handle and do not create a mess in the event of spillage. This is not the case with liquid color. Liquid color is difficult to handle and, in the event of a spill, creates a serious clean-up problem with a threat of a safety hazard, since liquid color material spilling on a factory floor, which is typically smooth concrete, creates a very slippery surface.
Coloring agents for plastic parts, whether the agents be solid colored pellets or liquid color, are expensive. Coloring agents are never underused since underusage results in a defective, undercolored part. Overuse of a coloring agent does not alter the color of a fabricated finished plastic part. Instead, overuse of a coloring agent results in much the same effect as painting a wall multiple times with the same paint—it does not change the color of the wall. Accordingly, some overuse of the coloring agent is almost always the case. However, overuse is to be avoided since overuse means waste of the coloring agent, with consequent costs that need not be incurred in fabricating the plastic parts of interest.
Liquid color, with the attendant potential for problems in the event of a leak or spillage of the liquid color material, is currently typically added at the throat of an extrusion process or directly into an end of a barrel of a molding machine. If liquid color is added further upstream, for example into the weigh bin of a gravimetric blender as disclosed in U.S. Pat. No. 6,007,236, there is risk of liquid color contamination of the blender parts, which may interfere with blender operation. Additionally, introducing liquid color to the process by adding the liquid color into the blender weigh bin runs the risk of incurring color changeover issues. These problems have, to some extent, restricted use of liquid color in the plastic molding and extrusion.
Liquid color diaphragm pumping apparatus for use in the plastic molding and extrusion is disclosed in published U.S. patent application Ser. No. 10/066,338, filed 31 Jan. 2002 and published as United States patent publication 2003-0142580, and in pending U.S. patent application Ser. No. 10/856,422, filed 28 May 2004 and published as United States patent publication 2005-0052945.
Lost of weight or “gravimetric” control has previously been used in processing solid color pellets furnished to plastic resin process equipment using auger feeders. However, gravimetric control has not heretofore been used with liquid color and associated liquid color pumps.